Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, a great deal of research and study has been focused on lithium secondary batteries having high-energy density and high-discharge voltage. These lithium secondary batteries are also commercially available and widely used.
The lithium secondary battery uses a metal oxide such as LiCoO2 as a cathode active material and a carbonaceous material as an anode active material, and is fabricated by disposition of a porous polymer separator between the anode and the cathode and addition of an organic electrolyte or polymer electrolyte containing a lithium salt such as LiPF6. Upon charging, lithium ions deintercalate from the cathode active material and intercalate into a carbon layer of the anode. In contrast, upon discharging, lithium ions deintercalate from the carbon layer of the anode and intercalate into the cathode active material. Here, the electrolyte serves as a medium through which lithium ions migrate between the anode and the cathode. Electrical energy is generated by oxidation-reduction reaction upon intercalation-deintercalation of lithium ions into and from the anode and the cathode.
In such a lithium secondary battery, the life characteristics and high-temperature storage characteristics of the battery are essential requirements that the battery must have. The battery using a conventional cathode active material has a disadvantage in that water and a lithium salt, e.g. LiPF6, present in electrodes or electrolytes, react to form a strong acid HF, which is accompanied by undesirable side reactions. That is, the thus-formed HF results in dissolution of cathode and anode active materials, thereby degradation of the electrode performance. In addition, HF leads to formation of lithium fluoride (LiF) on the cathode surface, consequently increasing electrical resistance, and gas evolution results in deterioration of the battery life. In particular, as a dissolution rate of the electrode materials caused by HF is increased at high temperatures, the formation of HF gives rise to significant problems associated with the battery cycle life and storage properties at high temperatures.
Further, when a cyclic carbonate having high polarity is used as an organic solvent of an electrolyte for the secondary battery, this may results in deterioration of the battery life characteristics due to an increased viscosity. To overcome such a disadvantage, U.S. Pat. Nos. 5,521,027 and 5,525,443 disclose a non-aqueous electrolyte of a linear carbonate having a low polarity but a low viscosity, mixed with a cyclic carbonate, in order to reduce the viscosity. Representative examples of the linear carbonates may include dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylmethyl carbonate (EMC). Among them, EMC having the lowest freezing point of −55° C. exerts superior low-temperature performance and life performance when it is used. As examples of the cyclic carbonates, mention may be made of ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC). Inter alia, PC has a low freezing point of −49° C. and thus exerts good low-temperature performance. However, when graphitized carbon having a large volume is used as the anode, PC is highly reactive with the anode during a charging process, and therefore it is difficult to use large amounts of PC. For this reason, EC, which forms a stable protective film at the anode, is primarily used. However, it cannot be said that EC is completely free of reactivity and therefore decomposition of the electrolyte, which occurs at the anode and cathode during charging/discharging of the battery, is one of numerous causes that shorten the battery life. In particular, EC exhibits highly increased reactivity at high temperatures, thus resulting in various problems.
As an attempt to overcome such problems and thereby improve the battery life at room temperature and high temperature, Japanese Unexamined Patent Publication No. 2000-123867 discloses a battery in which small amounts of ester compounds (for example, vinylene carbonate) having a cyclic molecular structure and C═C unsaturated bonds within the ring were added to the electrolyte. It is believed that such additive compounds decompose at the anode or cathode and then form films on the surfaces of the electrodes, thereby inhibiting decomposition of the electrolyte. However, such additives also cannot completely prevent decomposition of the electrolyte.
In addition, Japanese Unexamined Patent Publication No. 2002-25611 discloses a battery in which ethylene sulfite and vinylene carbonate were added to the electrolyte, and Japanese Unexamined Patent Publication No. 2002-270230 discloses a battery in which various kinds of ethylene sulfite compounds were added to the electrolyte. However, it was also confirmed that those additives disclosed in the above-mentioned prior arts did not exert a desired degree of effects.
Further, Japanese Unexamined Patent Publication No. 2000-323169 discloses a battery comprising an electrolyte with incorporation of a benzoic ester wherein hydrogen atoms of a benzene ring are substituted with one or more fluoro or trifluoromethyl groups. However, the additives disclosed in the above Japanese Patent are very expensive resulting in increased production costs of the battery and do not provide improvement of high-temperature characteristics to a desired degree. That is, since the electrolyte decomposition due to the graphitized carbon of anode is further accelerated at high temperatures, such additives cannot provide excellent improvement of high-temperature characteristics when the battery using those additives is used as a power source for devices requiring high-temperature operation, for example electric vehicles (EVs) and hybrid electric vehicles (HEVs).